This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Molybdenum is the only second-row transition element with a known function in living systems. It fulfills functional roles in enzyme systems in almost all living creatures, from bacteria through plants to invertebrates and mammals. The molybdenum enzymes all contain the metal bound by an unusual sulfur-containing cofactor. Despite possessing common structural elements, the enzymes are remarkable in the range of different chemical reactions that are catalyzed, although almost all are two-electron oxidation-reduction reactions in which an oxygen atom is transferred to or from the molybdenum. The functional roles filled by molybdenum enzymes are equally diverse;for example, they play essential roles in microbial respiration, in the uptake of nitrogen in green plants, in controlling insect eye color, and in human health. The proposed work is a systematic study of the major families of molybdenum enzymes using x-ray absorption spectroscopy. A new high-resolution approach will be used that allows the active sites to be described in much more detail. The ultimate goal of the work is to develop an in-depth quantitative structural and electronic understanding of the catalytic mechanisms used by the enzymes, and to compare and contrast them between and within the different families. From this we hope to improve our understanding of how molybdenum enzymes achieve their remarkable functional diversity.